Is this a full batch (3, 5, 6 gallon?) then you need to worry about oxidation for the rest of your brew. Even if you are careful while racking off your gallon, you are creating a ton of headspace, which is bad for any brew. You can top it off with another brew, more juice, a honey/sugar and water mix whatever. But keep in mind if you add more fermentables, the yeast will keep on working, which means more lees, and more time before its all going to be done. Having said that, when I did my blueberry cider (blueberry apple wine really at 12%ish) I siphoned off enough for 4 wine bottles, about 80% of a gallon, I took advantage of the headspace. I added more blueberries, some more apple juice, and some pure blueberry juice. This completely changed the brew, it's stronger, it's more complex, hell its just fantastic now. So you can always alter your brew now that you are creating this opportunity for yourself.
*Edit, I think I would let that girl do whatever she wanted any day of the week.